Derivatives of 2-hydroxybenzoic acid esters are useful starting materials for natural product synthesis or for the manufacture of fungicidal benzophenones such as those described in U.S. Pat. No. 5,773,663. Methods to prepare said 2-hydroxybenzoic acid esters are known, i.e. F. M. Hauser, et al., Synthesis 1980, 814 or Y. Hamada, et al, Tetrahedron, Vol. 47 (1991), 8635. However, these known methods require several steps and utilize corrosive or toxic reagents and are not amenable to large scale preparation or commercial manufacturing conditions.
The two-step syntheses cited in Synthesis and Tetrahedron hereinabove require the isolation of intermediates resulting in an undue solvent waste load on the environment. Further these syntheses require gaseous HCl and a separate oxidation procedure employing oxidizing reagents such as Br2 or CuCl2.
Therefore, it is an object of this invention to provide an effective and efficient single-step process to prepare 5- and/or 6-substituted-2-hydroxybenzoic acid esters which is amenable to large scale preparations and commercial manufacturing procedures.
It is another object of this invention to provide an effective means of obtaining a substituted-2-hydroxybenzoic acid ester in good yield under relatively mild reaction conditions from readily available starting materials and reagents.
These and other objects and features of the invention will become more apparent from the detailed description set forth herein below.
The present invention provides a single-step process for the preparation of a compound of formula I 
wherein R is C1-C6alkyl; and
R1 and R2 are each independently H or C1-C4 alkyl which process comprises reacting a compound of formula II 
wherein R is C1-C6alkyl and X is halogen or OCOCH3 with a compound of formula III 
wherein R1 and R2 are each independently H or C1-C4 alkyl, and formula III compound is understood to be cis, trans, or a mixture thereof, in the presence of a C1-C4carboxylic acid salt and a solvent, wherein the molar ratio of starting materials: compound III to compound II is higher or equal to 1.8.
Substituted-2-hydroxybenzoic acid esters of formula I are useful as key starting materials in natural product synthesis and in the manufacture of important benzophenone fungicidal agents. Therefore, the efficient preparation of such fungicidally active compounds in an environmentally sound manner is highly desirable.
The present single-step process makes salicylic acid of the formula I available from xcex2-ketoesters of the formula II and aldehydes of the formula III. Its practicability is ensured by using the aldehyde III in a molar ratio of not less than 1.8 relative to the compound II. This ensures that the reaction mixture remains efficiently stirrable even at low solvent quantities and consistently good yields are obtainable even on an industrial scale.
In a preferred embodiment of the process, the aldehyde III is initially charged and subsequently the salt and the solvent are added in succession or concurrently in the course of generally 0 to 3 hours.
It can further be of advantage to raise the reaction temperature in the course of the reaction from initially 60-120xc2x0 C. to finally 130-140xc2x0 C.
Preferred compounds prepared by the process of the invention are those compounds of formula I wherein R1 is C1-C4 alkyl and R is hydrogen. More preferred compounds are those compounds of formula I wherein R1 is methyl and R2 is hydrogen.
Preferred compounds of formula II employed in the process of the invention are those compounds wherein X is halogen. More preferred compounds are those compounds of formula II wherein X is Cl.
Compounds of formula III may be represented in the cis or trans configuration or as a mixture thereof. In the specification and claims, compounds designated as formula III include the cis isomer, the trans isomer or a mixture therof.
The term halogen as used in the specification and claims designates Cl, Br, F or I.
In accordance with the process of the invention, a xcex2-ketoester of formula II is reacted with an xcex1,xcex2-unsaturated aldehyde of formula III in the presence of a C1-C4 carboxylic acid salt and a solvent to form the desired product of formula I. The reaction is shown in flow diagram I wherein M is an alkali metal or an alkaline-earth metal 
Suitable solvents for use in the inventive process include polar solvents, preferably protic solvents such as C1-C4carboxylic acids or C1-C6alkanols. Preferred solvents are C2-C4 carboxylic acids or mixtures of a C2-C4 carboxylic acid and a C1-C6 alkanol, more preferably acetic acid or a mixture thereof with methanol or ethanol. In general, more than 2.5 molar equivalents of solvent; preferably about 2.5 to 5 and more preferably about 2.5 to 3.5 molar equivalents of solvent were used.
Acid salts suitable for use in the process of the invention are C1-C4 carboxylic acid alkali metal or alkaline-earth salts, more preferably acetic acid alkali metal salts such as sodium acetate or potassium acetate. Preferably about 1.3 to 3.0 molar equivalents, more preferably about 1.4 to 1.6 molar equivalents of the carboxylic acid salt were added to the reaction mixture.
In the present process, the aldehyde III is used in a molar ratio of not less than 1.8 relative to the compound II. Preferably about 1.9 to 2.5 and more preferably about 2.0 to 2.2 molar equivalents of aldehyde III were reacted.
In a preferred embodiment of the process, the aldehyde III is initially charged and subsequently the salt and the solvent are added in succession or concurrently in the course of generally 0 to 3 hours. It can be of advantage here first to add a portion or the total amount of the salt and only then to start with the addition of the solvent. Compound II is generally added last in the course of 0 to 3 hours.
In the process of the invention, reaction rate is directly related to reaction temperature, that is, the reaction rate increases with increased temperature. However, excessively high reaction temperatures may lead to decomposition and the formation of undesired by-products, thereby reducing product yield and purity. Suitable reaction temperatures in the process of the invention may range from room temperature to the reflux temperature of the solvent preferably about 60xc2x0 C. to 150xc2x0 C., more preferably about 110xc2x0 C. to 140xc2x0 C.
It can further be of advantage to raise the reaction temperature in the course of the reaction from initially 60-120xc2x0 C. to finally 130-140xc2x0 C. The reaction temperature can be raised for example by removing low boilers, if necessary by applying reduced pressure. It is similarly possible to carry out the reaction in a closed system. In this case, a higher reaction temperature will automatically result in a higher pressure. The pressure employed is preferably in the range from 0.1 to 6 atm.
The individual reactants are generally added at room temperature to the reflux temperature of the solvent.
The reaction time is generally in the range from 3 to 8 hours.
The formula I hydroxybenzoic acid ester product may be isolated using conventional isolation techniques such as precipitation, decantation, filtration, extraction, chromatographic separation or the like, preferably filtration or extraction.
Compounds of formula I are useful as intermediates in the synthesis of natural products in the manufacture of benzophenone fungicidal agents as described in U.S. Pat. No. 5,773,663.
For a more clear understanding of the invention, the following example is set forth below. This example is merely illustrative and is not to be understood as limiting the scope or underlying principles of the invention in any way.
The term NMR designates nuclear magnetic resonance spectroscopy. Unless otherwise mentioned, all parts are parts by weight.